A laboratory reports that an E. coli isolate produces an ESBL and is treated with piperacillin-tazobactam. Which mechanism specifically explains why ESBL-producing organisms may remain resistant despite a beta-lactamase inhibitor being present?
- A ESBLs have an altered active site serine that prevents tazobactam binding
- B Piperacillin is hydrolyzed by plasmid-encoded AmpC which tazobactam cannot inhibit, causing dual resistance
- C High ESBL enzyme burden (inoculum effect) produces more enzyme than the inhibitor can suppress at clinical concentrations ✓
- D Tazobactam is actively effluxed by MexAB-OprM pumps expressed by ESBL strains
Explanation
ESBL-producing organisms often fail piperacillin-tazobactam despite in vitro susceptibility, partly due to the inoculum effect: at high bacterial burdens present in deep-seated infections, the amount of ESBL enzyme produced can exceed the inhibitory capacity of tazobactam at achievable clinical concentrations, allowing residual beta-lactamase activity to hydrolyze piperacillin. Clinical outcomes studies (e.g., Rodriguez-Bano et al.) showed higher failure rates with pip-tazo versus carbapenems for bloodstream infections caused by ESBL producers. Additionally, some strains co-harbour AmpC or carbapenemases, but the inoculum effect is the primary explanation for clinical failure in ESBL-only producers.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.